1. Field of the Invention
The invention relates to a process and device for the manufacture of strip and composite bodies of metal in which a stream or streams of overheated metal melt is/are directed towards a surface moving transversely to the stream direction, producing an initially liquid metal film.
2. Discussion of the Prior Art
Processes for the rapid solidification of metals have recently gained increasing importance, since they permit the manufacture of new types of materials having partly improved or even unusual structures and, consequently, material properties. With an increasing solidification rate, an ever increasing deviation from the equilibria as determined by the equilibrium diagram occurs as a result, since the extremely short diffusion times impede the appearance of these equilibria. This leads on the one hand to continually finer morphology, e.g. to the development of finer dendrites or eutectics while the interdendritic or cellular segregation is reduced and in some materials can lead to the development of highly metastable structures and even to the formation of metallic glasses in an exceptional case. During the crystalline solidification there is an advantage here in the fact that the solubility range of certain desired elements is greatly widened, whereas undesirable precipitations can be suppressed.
The fundamental principle of all processes for rapid solidification is rapid heat extraction. This action is determined on the one hand by the thermal conductivity of the metal and on the other hand by the mechanism of heat transfer at the phase boundary to the heat-extracting medium. Whereas the heat transfer, characterised by the heat transfer coefficient, can be optimised in a wide range by selecting the correct process conditions, the heat transport in the metal, which is characterised by the coefficient of thermal conductivity, can only be improved by the selection of shorter transport paths. Therefore all currently known methods of rapid solidification lead to castings which have only a small thickness at least in the spatial direction of the heat transport. Examples of this are splat cooling, where a metal drop is abruptly transformed into a foil between two metal plates, the melt-spinning process, where a metal stream is usually applied to the outer surface of a rapidly rotating roll, a thin metal film being formed in a continuous manner under the effect of the acceleration as well as by the heat extraction of the roll, which serves as a quenching body, and certain powder-atomising processes, where a metal stream is beaten into small drops under the effect of an atomising medium which can be a gas or even a liquid, which drops solidify in flight and can subsequently be fed to powder-metallurgical compacting processes. The theoretical principles of processes for rapid solidification are clearly described, for example, in a publication by R. Mehrabian, "Rapid Solidification", reproduced in "Rapid Solidification Technology Source Book", American Society for Metals 1983, pp. 186-209. The most common processes can be gathered from chapters by G. Haour, H. Bode, "From Melt to Wire", and R. E. Maringer, "Payoff Decade for Advanced Material", from the same book, pp. 111-120 and pp. 121-128.
In the processes of spray compacting it is possible to produce larger cast structures, in which case semi-finished products can be produced in dimensions close to the final contours at higher cooling rates. Here, a melt, as a rule overheated by 50-150 K above the liquidus temperature, is usually atomised by means of argon or nitrogen as is the case in powder manufacture. During flight, a substantial portion of the excess heat is taken from the drops by the atomising gas so that the drops--in accordance with their size--strike the substrate in a more or less partially liquid state and weld there to the material deposited beforehand. The process is in principle suitable for the manufacture of flat products, but in particular for the production of rotationally symmetric semi-finished products such as round bars and pipes, the substrate in these cases performing a rotational movement with lateral offset during the spraying operation. Since the metal drops strike with only very low overheating, the substrate, i.e. the material already deposited beforehand, must be at a sufficiently high temperature so that homogeneous welding still occurs. However, if the temperature is too high, a liquid layer builds up on the substrate surface, which liquid layer on the one hand solidifies slowly in a conventional manner and on the other hand is thrown away from the substrate under the effect of the centrifugal force. Since the overheating of the sprayed-on metal particles is not constant as a result of their non-uniform grain-size distribution, the so-called overspray occurs anyway even at an optimum setting of the process parameters. This is the proportion of the spray particles which either fly past the substrate from the beginning or are thrown away from the latter as a result of too low a temperature. In particular in the case of expensive materials, this leads to an uneconomic yield, and in addition the fine metal powders deposited as a result in the spray chamber are in many cases dangerous as a result of their explosiveness and toxicity. Although spray compacting, compared with conventional powder metallurgy, has the advantage that all intermediate stages between powder atomising and powder compacting are dispensed with--and thus the chances of contaminating the powder surface are reduced--an enormous surface area is however still formed as in normal powder metallurgy and, in the case of highly reactive materials or even in the event of only slight contamination of the gas atmosphere in the spraying chamber, this can lead to damage to the material despite the short reaction times.
A considerable disadvantage of spray compacting consists in the fact that, although the cooling, taking place during the flight time, down into the range of the liquidus temperature takes place relatively quickly, e.g. at several thousand Kelvin per second, the subsequent cooling rate at the substrate, where the critical range between liquidus and solidus temperature is passed, is only in the order of magnitude of a few Kelvin per second. Thus the phenomena known from conventional solidification such as segregation as well as the formation of shrinkage cavities and precipitations are possible on the one hand, but so too is a coarsening of the original cast morphology. A further disadvantage of the process consists in the fact that the heat dissipation, as in all conventional solidification processes, takes place via the layers already solidified beforehand, whereupon the heat transport is reduced with increasing thickness of the substrate, which leads to non-stationary solidification conditions. On the other hand, a great advantage of the spray compacting process is that large quantities of metal in the order of magnitude of several kilograms per second can be converted, which makes the utilisation interesting within the scope of large production processes for semi-finished products. The processing aspects of spray compacting are clearly described in a paper by W. Kahl and J. Leupp "Spray Deposition of High Performance Aluminium Alloys via the Osprey Process" in Swiss Materials 2/4 (1990), pp. 17-19.